the coking properties of coal at elevated pressures. - Argonne ...

tion in this region may not be neglected.
In the following, model calculations are presented of the reduction of NO along
the height of the freeboard of the MIT 0.6 x 0.6m cross section, 4.5m high fluidized
bed. In the model a new approach is made to the prediction of solids concentration
in the freeboard (Chaung (15))which is assisted by measurement data on the descend-
ing flux of particles. The NO reduction along the height above the bed is then pre-
dicted from experimental information on the carbon content of bed solids and the
chemical kinetic rate equation on the NO-carbon reaction. The NO reductions so cal-
culated are then compared with measurement data obtained burning bituminous and sub-
bituminous coals in the 0.6 x 0.6m MIT experimental facility.
2.0 Particle Entrainment in the Freeboard
2.1 Theory of Particle Entrainment
A model has been developed for the entrainment of particles from the bed into
the freeboard of a fluidized combustor. Its purpose is to provide an estimate of
the total surface area of each solid species participating in chemical reactions in
this zone. The model is based on the following assumptions:
1. Bursting bubbles eject particles into the freeboard.
2. Particle-particle interactions and wall effects are negligible.
3. The changes in mass and size of a particle due to chemical reaction while
in the freeboard are negligible.
4. Interaction of the concentration, temperature, and velocity gradients
surrounding each particle are negligible.
5. The initial velocities of particles ejected from the bed are given by a
semi-empirical log-normal distribution having a geometric mean value proprotional
to bubble velocity.
6. Bubble diameters are determined by the heat exchanger tube spacing and the
bubble growth model of Mori and Wen (16).
The sequence of steps in the calculation of particle density (mass of par-
ticles/volume of gas-particle mixture) is as follows:
1. The initial velocities of the particles leaving the bed are given by the
assumed semi-empirical velocity distribution.
2.
3.
The initial flux of particles is proportional to the flux of bubbles at the
top of the bed. The proportionality factor is determined by equating the
kinetic energies of a bursting bubble and the particles which it ejects.
The equation of motion is solved for each particle size and initial velo-
city to determine the particle trajectories.
4. The particle density is determined from the ratios of flux to velocity at
each freeboard height. Total density is found by summing the contributions
from all sizes and initial velocities.
Because solution of the equations of motion is time-consuming, for the required
range of particle sizes and initial velocities, a simplified version of the model
was also developed, based on two additional assumptions:
7. Small particles, having ut < ug, ascend with constant velocity, vs = ug -
8.
Ut'
The drag force is negligible on particles having u
t L ug.
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